This invention relates to an imbalance detector for a centrifuge and, in particular, to an imbalance detector that is able to be tuned to correspond to the critical frequency of the rotor and rotor drive of the centrifuge with which it is associated.
A centrifuge is an apparatus adapted to subject a sample of a material to a centrifugal force field in order to separate that sample into its constituent components. For example, a common use for a centrifuge is in a bloodbank where whole blood received from donors is separated by centrifugal force into its red cell and plasma components. The centrifuge includes a rotating member, or rotor, mounted for rotational movement within a stationary casing. The rotor may be outfitted with buckets or cavities which carry the material to be separated during the period of exposure to the centrifugal force field. While the rotor is operating it is vital to be able to detect force imbalances which may be imposed upon the rotor. Such imbalances may occur, for example, as a result of unbalanced positioning of the load in the rotor. If left unchecked, the imbalancing forces may result in premature failure of the centrifuge drive.
Like any dynamic element the rotor has associated with it a critical frequency corresponding to a critical rotating speed through which the rotor must pass when moving from rest to its operating speed. The critical frequency and hence the critical speed of a particular rotor is dependent upon the physical configuration of the rotor as well as the stiffness of the rotor drive. It is at the critical speed that any imbalances in the rotor tend to cause maximum vibrational amplitudes of the rotor. Due to the self-centering flexible nature of the drive imbalance forces at critical speed are likely to be significantly greater than the imbalance forces imposed on the centrifuge while at its operating speed. Accordingly, if the imbalance forces imposed by a rotor on the centrifuge during its transit through its critical speed are below a predetermined threshold, it is likely that imbalances imposed at its operating speed will also remain below that threshold.
It is believed advantageous to provide a rotor imbalance detector for a centrifuge which is tuned to exhibit its critical frequency at or about the critical frequency of the centrifuge rotor. Thus, the imbalance detector is arranged to be responsive to the imbalance forces which are imposed by the rotor on the centrifuge through the drive while the rotor is at its critical speed and to generate a safety signal resulting in the occurrence of preventive action if the imbalance forces exceed a predetermined threshold.